The HP 8640B RF Signal Generator

The HP 8640B is fantastic piece of engineering and is a signal generator that is rugged, stable and exceptionally reliable. We specialize in test equipment at Electronics Revisited, LLC and have a particular expertise in several models of which the 8640B is one.

We are writing this to help those who may be interested in purchasing a used 8640B with the hope that this article will help you understand the strengths and weaknesses of this unit and help you ask the seller pertinent questions to make your purchase a good one.

The basic 8640B covered from 500 kHz to 512 MHz and the device can over range above and below these points (450kHz to 550MHz) . There are four basic options that are available. Option 001 allows the user to expand the AM modulation band up to 600 KHz. (The base model has two modulation frequencies, 400 Hz and 1 kHz) Option 002 expands the output frequency with an internal doubler up to 1024 MHz. Option 003 provides for up to 50 Watts of reverse power protection. Option 004 is primarily for avionics testing.

Regarding option 002, although the base model only goes up to 512 MHz, the front panel dial will have a last detent of "512-1024" MHz. Buyer beware here, all the base models have this frequency range. If option 002 is not included, this section of the range has a black background, if option 002 is included, this will appear on a white back ground AND option 002 should appear stamped on the rear options label. When the base model (without opt 002) is switched to this high range, the display will automatically double, this is for convenience only as it is assumed by HP that you are using an external doubler if the internal option 002 isn't installed. In addition, the maximum FM deviation ranges are also extended.

Option 004 which was designed primarily for working on aircraft avionics. It primarily offered expanded and precise AM depths to test VOR, ILS and COM radios. It also offered a 1 dB output step attenuator in addition to the standard 10dB steps.

Things to look for.........

If considering a purchase, look closely at the units front panel. All models of the 8640B have the ability to select internal or external AM or FM modulation. The switches for this selection are located near the left middle of the unit. Look closely at the pictures. These switches (same with the RF power on/off switch on the far right) was a weak point in the 8640B design. The small lever arms of these switches are easily broken if the unit is dropped or abused. If you can't see the small paddle arm of the switch, then it is likely broken and you won't be able to fix it without very extensive removal of internal components. If the arm is intact, it will likely remain so. It is only broken with rough handling. Likewise, look closely at all the remaining controls and inspect pictures for cracked or missing control knobs.

If a picture of the rear panel is available from the seller, note if the two plastic rear feet are visible on the right side of the unit. It is not uncommon for one or both of these to be broken off. They are located just to the left of the power port connection. This is not a problem if you are using this as a bench unit. The rear feet only help stabilize the unit if it is tipped up on its back end. There may or may not be up to five feet on the bottom of the unit. Again, not a critical point if they are missing and are specially designed so that you can stack HP equipment one on top of the other. ( We generally remove the feet on any HP equipment we have in house. We find they tend to get in the way more than anything.) The only advantage that we have found with the feet is that some models may come with a tilt stand in the front which attach to the front two feet; a nice feature if you don't have a test equipment bench or shelving in front of you on which to rest the unit.

Another limitation of this device is that there are several incandescent bulbs on either side of the power meter which indicate the meter scale (left side) and various over-range (right side) conditions. It is not unusual for one or more of these bulbs to burn out. You really can't tell from a picture if the bulb is burnt because they only illuminate under the correct conditions. When we refurbish a unit, we will replace these bulbs with bulbs with a slightly higher voltage rating to insure long life. Make sure that potential sellers have tested and reported as working all six bulbs.

The 8640B uses a mechanically tuned cavity oscillator to set a very stable frequency output. The tuning assembly sits directly behind the tuning knob. If the inside of this assembly is dirty or contaminated, the result may be that the unit will drop out of frequency, usually at one end or the other of the limit stops. Again, ask the seller to guarantee that the frequency adjustment knob operates over its entire range. Complete tuning across the entire selected band is accomplished with about eight turns of the tuning knob. There is a fine tuning adjustment just to the right of the main knob which adjusts to 200 ppm. Restabilization time after tuning is very short. There is also a phase-lock button which will lock in the frequency and also allow for a 1/2 of scale extension of accuracy. (1/2 of least significant digit)

Many of these units may have minor dents and dings in the case, this is usually not a problem. HP designed these units well and they are mechanically robust. Dents and dings should not pose a problem.

A weakness we occasionally see in the 8640B design was the use of plastic gears with brass inserts in the tuning, FM and band selection mechanics. The plastic may shrink over many years, yet the brass insert doesn't. The result is a cracked gear. Symptoms can be anything from a bad frequency display to a completely inoperative unit. Each gear has two set screws; the cracks most always occur near the set screw holes. We check and/or rebuild all gears if there are through cracks which could lead to an inoperable condition in the future. The gears are in general extremely robust and we've seen units with bad cracks work flawlessly but we fix or replace them in any case.

The only other minor weakness of note are the contact fingers on some of the custom rotary switches. The fingers are bifurcated spring metal which are attached to the plastic wheel with plastic rivets. The rivets can loosen over time and the contacts will no longer work. These can be easily fixed with some plastic cement and/or epoxy. All of our 8640B's are fully inspected in this area and repaired as needed.

It is always best if you can spot a picture when the unit is turned on and both the frequency meter and the analog meter is displaying a value. Ideally, the analog meter should be set to display the output power, however it may be difficult to discern if the output button is depressed versus the modulation buttons. (One meter, three selector switches) The frequency display on the 8640B is a stand alone frequency counter which can send a phase lock signal back to the oscillator to lock in the signal. If the digital display is showing a value which corresponds to the frequency range selector this assures you that the oscillator is operating and that the range selector switch is probably operating. If the meter is set to read power output and it is also displaying a value, you are now very assured that the basic operation of the oscillator, frequency counter and power output stages are good. What remains is the AM and FM modulation sections and a reputable seller will guarantee and perhaps offer a warranty on the unit. (Warranties are available on almost everything we sell. If we don't offer a warranty, we will at least offer full disclosure.)

I have highlighted some of the weaknesses and strengths of this model. There are many other high points that are too numerous to list. Various versions of this signal generator were produced with slight nuances such as display resolution, 1/2 digit display in phase-lock mode, changes in the knob design, internal changes in the selector switch designs, etc.

The 8640B is a very durable and reliable piece of test equipment. Parts or equivalent substitutions are still available and any problems may be easily diagnosed down to the component level. There are many sources of schematics and operations manuals available in the public domain. A well maintained 8640B should offer you a long and reliable life. Your best defense is to purchase from a good and established seller who can offer you at least a guarantee of functionality and perhaps a warranty on any unit you buy. The sellers feedback score, feedback comments and detailed seller ratings are an excellent way to gauge a sellers reputation.

If you have any questions on the 8640B please don't hesitate to email us. Even if you're not purchasing from us we are still more than willing to help you with tips and questions, where to locate documentation, etc.